The words ileum, ilium sound the same but have different meanings and spellings. Why do ileum, ilium sound the same even though they are completely different words?
The answer is simple: ileum, ilium are homophones of the English language.
The terminal portion of the small intestine extending from the jejunum to the cecum.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.